ABSTRACT Young adult drinking most frequently occurs in a social facilitation context, in groups and with close friends, and such situations are high risk for excessive alcohol use. Ecological momentary assessment (EMA) has been used to gather information about drinking episodes, but relying on participants to initiate reports under peer-intensive circumstances has limitations. A promising means to detect and characterize social drinking contexts is to use an unobtrusive sensor-based approach, a Bluetooth-based transmitter, in the context of an EMA protocol. In this application we propose to develop the optimal methods for using Bluetooth devices to detect contact between young adults in order to gather real-time information about high-risk social drinking situations. The project?s aims are to: (1) establish the feasibility and acceptability of an EMA protocol using Bluetooth-based detection of contacts among participants and their peers; (2) evaluate the Bluetooth technology with regard to functionality, as measured by user experience and app data, and reliability, as measured by the concordance between Bluetooth detection of social contact and participant self-report; and (3) evaluate the peer selection process with regard to the predictive utility of peer characteristics for interactions with alcohol involvement and to evaluate the moderating role of peer characteristics on the feasibility, acceptability, functionality, and reliability of the Bluetooth technology. Following a pilot phase with a small (N = 5) group of participants to adjust procedures and technologies, a community sample of 35 young adult drinkers (age 18-24) will complete a three-week EMA study. Participants first will complete a social network interview to assess characteristics of close peer relationships (quality, frequency of interaction, frequency of consuming alcohol with the peer). The participant will identify three peers who will be asked to carry a small ID-coded Bluetooth beacon or download an app (peers will not otherwise participate in the EMA phase). The EMA protocol will include signal-contingent prompts when the peer Bluetooth signal is detected, which will prompt a survey about the social context (including presence and drinking behavior of peers), as well as random prompts, drinking event-contingent surveys, and morning surveys. After the EMA phase, a follow-up interview administered to participants and peers will assess user experience (feasibility, acceptability of methods). Analyses will include enrollment, compliance, and retention of participants and peers, functionality of the Bluetooth approach with special attention to missing data, concordance between sensor and self-report data, and analysis of how different peer characteristics predict these outcomes. The primary objective of this innovative proposal is to obtain data on user experience and Bluetooth sensor usability to develop cutting-edge procedures, measures, and technology to support a larger investigation of drinking social context and provide a basis for the development and timing of just-in-time adaptive interventions that target youth alcohol use in the natural environment at a time when behavior is opportune for modification.